Liquid crystal display (LCD) panels typically comprise two pieces of thin glass (color filter substrate and TFT substrate) including a liquid crystal material disposed therebetween, a polymer light guide (e.g. Poly(methyl methacrylate)—PMMA) and a few thin plastic films (diffusers, DBEF films, etc.). Due to the poor elastic modulus of PMMA, the overall structure may have insufficient rigidity to withstand significant physical shock. Consequently, additional mechanical structure may be needed to provide stiffness. Young's modulus for PMMA is about 2 gigaPascal (GPa) for PMMA. In contrast, most silica-based glasses have a young's modulus of about 72 GPa.
Humidity testing shows that PMMA is extremely sensitive to moisture, and dimensions can change by about 0.5%. On a length of about a meter, that means about 5 mm of dimension increase, which is very significant and makes mechanical design of a backlight unit challenging. Typically, an air gap is provided between the light source (e.g. light emitting diodes—LEDs) and the PMMA light guide to accommodate expansion of the PMMA. Unfortunately, light coupling between the light source and the PMMA light guide is sensitive to the distance between the light source and the light guide, which can cause the display brightness to change as a function of humidity.
PMMA has a coefficient of thermal expansion (CTE) of approximately 75×10−6/° C. and has relatively low thermal conductivity of about 0.2 W/m/K, while the same attributes for temperature of about 105° C. The low thermal conductivity of the PMMA impedes heat dissipation from the material. Thus, as the light guide approaches close to the LED's, which can dissipate a lot of energy, the PMMA can become very hot.
Due to the poor thermal, mechanical and dimensional stability of PMMA, a glass, such as a silica-based glass, is considered a potential replacement solution for PMMA in liquid crystal display (LCD) backlight units. However, since high quality optical-grade glass can be more expensive to produce than PMMA, the cost of the glass drives technology development toward thinner light guides, while the size of the LED's that must couple to the light guide, for example at least 2 millimeters thick, require the glass to be thicker than traditional 0.07 millimeter (mm) thick LCD glass to enable close to 100% coupling of light from the LEDs into the light guide.